1. Field of the Invention
The present invention relates to a semiconductor laser apparatus, and more particularly, to a semiconductor laser apparatus which has a NAM (nonabsorbing mirror) structure for providing a high-output semiconductor laser of high reliability.
2. Description of the Prior Art
FIG. 7 is a sectional view showing a conventional semiconductor laser apparatus having a coupled waveguide disclosed in, for example, "A New Transverse-Mode Stabilized GaAlAs Laser with a Slab-Coupled Waveguide Grown by MOCVD", Extended Abstracts of the 16th (1984) International Conference on Solid State Devices and Materials, Kobe, pp. 153-156. The structure of this semiconductor laser apparatus will now be described. Referring to FIG. 7, an n-type GaAs substrate 1a is provided thereon with an n-type AlGaAs clad layer 2a (composition ratio of Al to Ga is, e.g., 45% : 55%), an undoped AlGaAs active layer 3a (composition ratio of Al to Ga is, e.g., 9% : 91%), a p-type AlGaAs clad layer 4a (composition ratio of Al to Ga is, e.g., 45% : 55%) and an n-type GaAs current blocking layer 5 in the said order, and a stripe channel 20a is formed on the p-type AlGaAs clad layer 4a and the n-type GaAs current blocking layer 5. Further, a p-type AlGaAs waveguide layer 6 (composition ratio of Al to Ga is, e.g., 35% : 65%), a p-type AlGaAs clad layer (composition ratio of Al to Ga is, e.g., 45% : 55%) and a p-type GaAs contact layer 8 are formed in the said order partially on the p-type AlGaAs clad layer 4a and on the n-type GaAs current blocking layer 5, and another stripe channel is formed on the p-type AlGaAs waveguide layer 6 and the p-type AlGaAs clad layer 7 correspondingly to the strip channel 20a. An upper electrode 9 is formed on the p-type GaAs contact layer 8 and a lower electrode 10 is formed under the n-type GaAs substrate 1a.
A method of manufacturing the semiconductor laser apparatus as shown in FIG. 7 will now be described. The said layers from the n-type AlGaAs clad layer 2a to the n-type GaAs current blocking layer 5 are epitaxially grown successively on the n-type GaAs substrate 1a by a chemical vapor deposition method called as an MO-CVD (metalorganic chemical vapor deposition) method. Then an open stripe resist mask is formed on the n-type GaAs current blocking layer 5 through photolithography. Thereafter the p-type AlGaAs clad layer 4a and the n-type GaAs current blocking layer 5 are chemically etched by a vitriolic etching solution or the like through use of the resist mask, to form the stripe channel 20a. In this case, the etching time is so controlled that the bottom portion of the stripe channel 20a is separated by 0.3 to 0.5 .mu.m from the undoped AlGaAs active layer 3a. Then the said layers from the p-type AlGaAs waveguide layer 6 to the p-type GaAs contact layer 8 are epitaxially grown partially on the p-type AlGaAs clad layer 4a and on the n-type GaAs current blocking layer 5 again by the MO-CVD method. Finally the n-type GaAs substrate 1a is polished to about 100 .mu.m in thickness for facilitating cleavage, and the upper and lower electrodes 9 and 10 are formed on the p-type GaAs contact layer 8 and under the n-type GaAs substrate 1a respectively.
Description follows on the operation of the semiconductor laser apparatus as shown in FIG. 7. When voltage is applied between the upper and lower electrodes 9 and 10 forwardly to a p-n junction formed in the interface between the undoped AlGaAs active layer 3a and the p-type AlGaAs clad layer 4a, a forward current through a region of the stripe channel 20a, from which the n-type GaAs current blocking layer 5 is removed, is injected into the undoped AlGaAs active layer 3a to cause light emission. The light is guided by a coupled waveguide defined by difference in refractive index between the n-type and p-type AlGaAs clad layers 2a and 4a and the undoped AlGaAs active layer 3a and a difference in refractive index between the p-type AlGaAs clad layers 4a and 7 and the p-type AlGaAs waveguide layer 6. The p-type AlGaAs waveguide layer 6 is bent by the stripe channel 20a, whereby a horizontal difference in refractive index is effectively caused in the growth layers, to stabilize the transverse mode. The light guided by such a coupled waveguide is applied to laser oscillation by a Fabry-Perot type resonator formed by opposite cleavage end planes perpendicular to the longitudinal direction (perpendicular to the figure) of the stripe channel 20a.
In the conventional AlGaAs semiconductor laser apparatus, the cleavage end planes serve as regions for absorbing laser light due to the surface states. Thus, the maximum optical output is defined by COD (catastrophic optical damage) at the cleavage end planes, whereby high output operation is restricted. Further, reliability of the semiconductor laser apparatus is damaged by gradual degradation (e.g., degradation in optical output-to-current characteristics) caused by oxidation of the cleavage end planes facilitated by heat generation upon absorption of the laser light.